Prostaglandin endoperoxidase synthase 2 is a key regulator of inflammation and may play a role in prostate carcinogenesis. The polymorphism, –899G>C (rs20417), alters a transcription factor-binding site and is associated with a reduced risk of colorectal adenoma. We tested the hypothesis that rs20417 may influence prostate cancer risk, using a large case–control study (ncases=1608, ncontrols=3058). We found no evidence that rs20417 alters prostate cancer risk (odds ratio (ORCC & GC v GG=1.05, 95% confidence interval (CI)=0.91–1.20). A meta-analysis of three studies also found little evidence that rs20417 alters risk (pooled ORCC & GC v GG=1.04, 95% CI=0.93–1.17), making it unlikely that rs20417 contributes in any major way to prostate cancer aetiology.
Prostate cancer is a major cause of morbidity and mortality in the United Kingdom1 and worldwide.2 Family and twin studies have provided strong evidence for an inherited predisposition to the disease.3, 4
There is also emerging evidence that chronic inflammation is important in prostate cancer aetiology,5 and the role of prostaglandin endoperoxidase synthase 2 (PTGS2) is under scrutiny. PTGS2 encodes cyclooxygenase-2 (COX-2), which represents the rate-limiting step in the biosynthesis of prostaglandins, powerful inflammatory mediators.6 Multiple studies have shown an increased expression of COX-2 in prostatic tumours7, 8, 9, 10 and a protective effect of the COX-inhibiting non-steroidal anti-inflammatory class of drugs,11 providing support for a role in prostatic carcinogenesis.
There is evidence that rs20417, located in the promoter region of PTGS2, affects COX-2 function. Possession of the C allele eliminates a nuclear factor-kappa B (NF-κB) binding site upstream of the translation initiation site.12 In vitro experiments using the human endothelial,13 brain,14 colonic15 and bronchial16 tissue have shown that intact NF-κB signalling pathways are critical to COX-2 upregulation. This may explain why patients with one or two C alleles show a reduction in COX-2 promoter activity17 and reduced expression of COX-2 in colorectal tumours.18 In line with these data, these alleles seem to reduce the risk of colorectal adenoma among certain populations.19
We hypothesize that by eliminating an NF-κB binding site critical to COX-2 expression, rs20417 can reduce inflammation and thereby protect against prostate cancer. To test this hypothesis, we present data from a large, case–control study nested within the Prostate testing for cancer and Treatment (ProtecT) trial and a meta-analysis of earlier published studies.
Materials and methods
Nested case–control study within ProtecT
The ProtecT trial is an ongoing, multi-centre, randomized controlled trial that will compare the efficacy, cost-effectiveness and acceptability of treatments for localized prostate cancer. Between 2001–2008 over 100 000 men aged 50–69 years (from nearly 300 general practices across the UK) were invited to prostate check clinics where histologically confirmed prostate cancer cases were identified through a combination of prostate-specific antigen (PSA) testing, digital rectal examination (DRE) and 10-core transrectal ultrasound-guided biopsy. All participants with no evidence of prostate cancer after PSA testing, DRE and/or biopsy were eligible to be controls. Controls were stratum matched to cases by age (5-year bands) and general practice. The index date for controls was the date of the prostate check clinic. Such matching automatically matches for calendar time, as prostate check clinics were completed sequentially. Detailed descriptions of ProtecT and the protocol for nested case–control selection are published elsewhere.20, 21, 22
Consent for genotyping and blood samples was taken from all patients enroled in ProtecT under the auspices of the Prostate Mechanisms of Progression and Treatment (ProMPT) collaborative and approved by Trent Multicentre Research and Ethics Committee. DNA extraction was carried out by Tepnel, Manchester, UK. (http://www.tepnel.com). Genotyping for single nucleotide polymorphism (SNP) analysis was carried out by KBiosciences, Hertfordshire, UK. (www.kbioscience.co.uk), whose personnel were blinded to patient status, using their proprietary KASPar PCR technique and Taqman Genotype calling was carried out using an automated system, the results of which were checked manually by study personnel using SNPviewer software, KBiosciences. Investigators reviewing genotyping data were blinded to patient status.
To further ensure adequate quality control, we compared genotypes obtained for this study with the results of genotyping carried out at an earlier date as part of a pilot study in 216 patients. Concordance for these 216 ‘blinded’ replicates was 100%. In addition, within our study, 412 duplicate samples were included. Concordance for these 412 ‘unblinded’ replicates was 100%.
Data from other studies
Relevant studies were identified by searching Medline and Embase bibliographic databases from their inception up to 1 December 2008 using the search terms: (‘prostaglandin endoperoxide synthase’ or PTGS or PTGS2 or COX or COX2) and (‘prostate cancer’(MeSH) or ‘prostatic neoplasm’(MeSH)). Further publications were identified through review of the bibliographies of retrieved articles. Studies that investigated associations between rs20417 and prostate cancer were eligible for inclusion. Where necessary corresponding authors were contacted for relevant data. No papers were excluded on the basis of language.
A Pearson χ2-test was carried out among controls to ensure the genotype distribution satisfied Hardy–Weinberg equilibrium. In ProtecT, ORs and 95% CIs for the association of rs20417 with prostate cancer were calculated using conditional logistic regression to account for the stratum matching of cases to controls. To assess whether rs20417 could be associated with progression rather than incidence of prostate cancer, we also calculated ORs for the association of rs20417 with advanced (T3–T4 or N1 or M1) versus localized (T1–T2, NX or NO, MX or MO) cancers using unconditional logistic regression adjusted for age and centre.23 To assess the potential for PSA detection bias,24 associations of rs20417 with mean serum PSA levels by genotype were investigated using t-tests. Study-specific ORs were meta-analysed using fixed (Mantel-Haenszel) models but random (DerSimonian and Laird) effect models were computed to confirm that similar results were obtained. All analysis was carried out using Stata 10 software (StataCorp, College Station, TX, USA).
Blood samples from 4666 participants were submitted for genotyping. We received genotypes for 1592 of 1608 cases (99.0%) and 3028 of 3058 controls (99.0%). Genotypes among controls were in Hardy–Weinberg equilibrium (P=0.53). The age, body mass index, social class and smoking status of cases and controls was similar (data available upon request).
In the ProtecT nested case–control study, we found little evidence that rs20417 altered the risk of developing prostate cancer (Table 1). Restricting analysis to localized cancers, also provided little evidence that rs20417 affects prostate cancer risk when compared with controls (ORCC & GC v GG=1.05, 95% CI=0.89–1.23). We found no evidence that genotype affects risk of developing advanced versus localized cancer (ORCC & GC v GG=1.05, 95% CI=0.62–1.78). rs20417 was not associated with mean PSA concentration among patients with prostate cancer or controls (data available on request).
Our literature search identified two relevant studies.12, 25 Only one of these studies12 contained information on the clinical characteristics of its case–control populations. Combining ProtecT data with these studies provided no evidence that rs20417 alters prostate cancer risk (Figure 1). Results under random effects models were identical to fixed effects models. Both of these studies included separate analyses of ‘African–Americans’. We combined these data with the data from ProtecT participants who self-reported as ‘African–Caribbean’ and found little evidence for an association between rs20417 and prostate cancer risk among this subgroup (ORCC & GC v GG=1.46, 95% CI=0.92–2.34, under fixed effect model).
Despite the putative functional effect of rs20417 on the inflammatory pathway, our large case–control study demonstrates no evidence to support the hypothesis that this genotype alters risk of prostate cancer development. In doing so, we suggest that forthcoming research on the aetiology of prostate cancer need not concentrate on this SNP.
Our findings differ from one earlier published case–control series reported by Panguluri et al.,12 which concluded that rs20417 is associated with an increased risk of prostate cancer among African–Americans. Such a conclusion runs contrary to our hypothesized expectations, which we based on the knowledge about the function of rs20417, and may have arisen by chance in an underpowered (small sample size) study. An inability to replicate earlier published studies based on modestly sized data sets is not uncommon.26
Our null results are unlikely to have been influenced by survivor bias because the majority of prostate cancer cases within ProtecT are localized, having been identified through population-based PSA testing rather than having been clinically detected at older ages. Another strength of our study is its large size, which provided over 90% power to demonstrate a change in prostate cancer risk of 20% at α=5%, although we cannot exclude either the possibility of a very modest effect of rs20417 on prostate cancer risk or potentially important associations among ‘African–Caribbeans’ and for advanced cancers. As we investigated just one SNP, albeit in a hypothesis driven manner, we cannot exclude a role for other SNPs or other types of genetic variation within PTGS2 in prostate cancer aetiology.
In conclusion, despite the putative importance of COX-2 in prostate cancer carcinogenesis and the supposed effect of rs20417 on its upregulation, our large, case–control study and meta-analysis has found no evidence that it influences prostate cancer risk. We suggest that future scientific interest in factors associated with prostate cancer risk should focus on other SNPs relevant to the inflammatory pathway and that larger prostate cancer case–control series of advanced disease and ethnic-specific populations are established.
Conflict of interest
The authors declare no conflicts of interest.
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The authors would like to acknowledge the tremendous contribution of all members of the ProtecT study research group, and especially the following who were involved in this research (Jane Athene Lane, Prasad Bollina, Sue Bonnington, Debbie Cooper, Angie Cox, Michael Davis, Liz Down, Andrew Doble, Alan Doherty, Emma Elliott, David Gillatt, Pippa Herbert, Peter Holding, Joanne Howson, Mandy Jones, Roger Kockelbergh, Howard Kynaston, Teresa Lennon, Norma Lyons, Hilary Moody, Philip Powell, Stephen Prescott, Liz Salter and Pauline Thompson). The ProtecT study is funded by the UK NIHR Health Technology Assessment Programme (projects 96/20/06, 96/20/99). The authors would like to acknowledge the provision of additional epidemiological data by the NHS R&D Directorate supported Prodigal study and the ProMPT collaboration, which is supported by the National Cancer Research Institute (NCRI) formed by the Department of Health, the Medical Research Council and Cancer Research UK. Genotyping was funded by the World Cancer Research Fund.
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Murad, A., Lewis, S., Smith, G. et al. PTGS2–899G>C and prostate cancer risk: a population-based nested case–control study (ProtecT) and a systematic review with meta-analysis. Prostate Cancer Prostatic Dis 12, 296–300 (2009). https://doi.org/10.1038/pcan.2009.18
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